In a large bowl, cream together coconut oil and almond butter with a whisk or an electric mixer. Add coconut sugar, flax, almond milk, and vanilla and mix again to incorporate.
In a medium bowl, stir together oat flour, salt, and baking powder. Add the crushed graham crackers.
Add the dry ingredients to the wet ingredients and stir to combine.
Preheat oven to 350 F and line a 8 inch by 8 inch square baking dish with parchment paper, leaving some overhang around the edges to allow you to easily lift the bar out. Divide the dough in half. Press half of the dough into the parchment paper-lined baking dish and carefully lift out. This will be your top layer. Set aside until ready to use. Line the pan with another sheet of parchment paper and press in the second half of the dough, pressing outward with your fingers to create a nice even layer. Top the dough with the chopped dark chocolate and marshmallows. To add the top layer of dough, carefully lift it and flip it over on top of the marshmallow layer. Gently press it down.
Bake for 25 minutes. Let it cool for at least an hour, preferably 2 hours, before slicing into 12 squares. Store in an airtight container at room temperature or in the refrigerator to extend shelf life.